The present invention relates to a method of synchronizing fin fold-out on a long-range artillery shell which is fin-stabilized on its trajectory towards the target and is intended to be fired from a rifled barrel and is to this end provided with a sliding driving band as the main contact surface against the inside of the barrel and also with a number of stabilizing fins which can be folded out after the shell has left the barrel. The purpose of the sliding driving band is to allow the shell, in spite of the rifling of the barrel, to leave the latter with only low rotation or no rotation at all.
It is particularly characteristic of the method and the shell according to the invention that the stabilizing fins of the shell are interconnected by specially designed movement transmission means which bring about uniform fold-out of all the fins irrespective of how these are loaded during the fold-out phase itself Even if the shell should leave the barrel entirely without rotation, the fins arranged around the shell will nevertheless be loaded differently during the fold-out phase by the forces generated by the air flowing past. This is because it has proved to be impossible to avoid any type of shell being subjected to a certain conical yawing motion on its trajectory, and this yawing motion begins immediately after the shell has left the mouth of the barrel.
The reason why an artillery shell is fin-stabilized instead of being rotation-stabilized may be, for example, that it is desirable to make it guidable on its way towards the target, and it is considerably easier to correct the course of a fin-stabilized shell than of a rotation-stabilized shell, and this is the case irrespective of whether the course correction concerned is intended to be performed by impulse motors, steering rudders or in another manner.
It is a requirement of the shell according to the invention that it should be capable of being given an extra long range. A method used increasingly in recent years of achieving extremely long ranges even in older barrel-type artillery is the base-bleed technique, which is used in order to eliminate the turbulence and negative pressure which are formed behind the shells flying through the atmosphere and have a braking effect on the shells and shorten their flying distance. The base-bleed technique is based on arranging a combustion chamber in the rear part of the shell, which chamber is filled with a slow-burning pyrotechnic composition which, while it burns, produces combustion gases which are allowed, in a predetermined quantity, to flow out through an opening in the rear end wall of the shell and there eliminate and fill the abovementioned braking turbulence and negative pressure behind the shell.
When a shell is to be provided with both a base-bleed unit and stabilizing fins, however, it is easy for positioning problems to arise, because the base-bleed unit definitely has to be arranged in the rear part of the shell with at least one gas outflow opening in the rear end wall of the shell, while the fins too ought to be positioned in the rear body of the shell as far away as possible from the centre of gravity of the shell, that is to say fins and base-bleed unit should preferably be arranged within the same part of the shell. An additional problem is that, in order to allow firing of the shell from a rifled barrel, the fins must be fully folded in inside the minimum diameter of the barrel during firing, at the same time as they must not occupy too great a volume either and thus prevent the use of this space for other purposes such as, therefore, the base-bleed unit or payload.
In a known type of fold-in fin, which takes up little space and can be designed so that, in the folded-in position, the fins can share the rearmost part of the shell with a base-bleed unit, each fin consists of a plate which is fixed to a rotatable spindle arranged in the longitudinal direction of the shell and which, in the folded-out position, will constitute the active area of the fin and, in the folded-in position, is rotated in towards the shell body about its spindle, and is in this position curved in towards the shell body and, until the desired fold-out time, is retained in this position by a protective cover or equivalent. Previously, such fins were designed with a curved shape following the shell body and they retained this shape in the folded-out position as well, but, in recent years, elastically deformable materials have become available, which have such a good shape memory that it is now possible to produce fins which, even after years of incurvation in the folded-in position, essentially recover their original shape. It has therefore become possible to use these materials to produce fins which, as soon as they are given the opportunity, tend to recover the shape they were originally given, and this may have been entirely plane or slightly propeller-shaped or designed in another way so as to be provided with a limited angle of attack relative to the air rushing past. One way, which is relatively simple in terms of manufacture in this context, of giving the fins the desired angle of attack is to provide them with a sharp or gently curved dog-ear design or a few degrees of propeller twist. All these types of guide fins are presupposed at the same time to have a radial main direction seen in the cross-sectional direction of the shell. The angles of action relative to the air rushing past the shell which are chiefly of interest in the case of the guide fins for fin-stabilized shells are usually of the order of 1-2.degree., and corresponding angles of action can of course also be brought about by means of axes of rotation for folding in and folding out the fins which are inclined relative to the longitudinal axis of the shell, but this would as a rule involve more expensive overall solutions.
As an example of the state of the art, WO 98/43037 may be mentioned, in which a fin-stabilized artillery shell with fold-out stabilizing fins of the type described above is disclosed.
In the introduction, it was stated that every type of artillery shell is already subjected to a certain form of conical yawing motion on the trajectory immediately after it has left the mouth of the barrel and that this results in fold-out fins arranged on the shell being subjected to different degrees of loading by the relative wind of the surrounding air, which can moreover, to some extent, be from different directions. In brief, this means that the various fins on a fin-stabilized artillery shell will be loaded differently during the fold-out phase itself. In the case of shells provided with sliding driving bands, the centrifugal force acting on the fins is of little importance for fin fold-out. Instead, the majority of the fold-out force comes from the straightening force of the fin material, that is to say the force which is generated when the elastic deformation of the fin material returns to the original shape the fin was once given. In their folded-in position, elastically deformed fins of the type concerned here will quite simply spread out by virtue of their own built-in force but, in spite of this, the fold-out function cannot be left entirely to this mechanical energy development, inter alia because it is clearly most marked during the initial introductory phase of fold-out. For this reason, the fins are normally also provided in the previously indicated manner with a small angle of attack relative to the flying direction of the shell, so that the forces of the air will, above all in the final stage of fold-out, make their contribution to the requisite fin fold-out force. However, on account of the yawing motion of the shell, the air forces may vary quite considerably in strength and direction between the different sides of the shell because the relative wind against the shell is dependent on the yawing motion of the shell which begins directly outside the mouth of the barrel. A fin on one side of the shell could therefore, if it were able to define its own fold-out speed, have such a high fold-out speed that its strength is put at risk, while a fin on another side of the shell could at the same time have such a low fold-out speed that it does not completely reach its intended radial position.